It's only certain models apparently, but it doesn't state which. See here for MBP (here for everybody else).

If you don't get it in Software Update, you most likely don't need it. If you're skeptical, download it manually and see if your system takes it (it will tell you it's not meant for your system if it isn't).

The MacBook disk image from the Apple support download page appears to be damaged - I got an error "There may be a problem with this disk image. Are you sure you want to open it?". I was able to examine the package after clicking the Open button, but it might be safer for MacBook users to get this one via Software Update until Apple fixes the image. The other two disk images mounted OK.

Edit: it occurs to me that the bad image might be a problem on my nearest Akamai mirror. Anyone else have problems mounting the MacBook disk image? (I downloaded it twice, didn't help.)